Nondestructive testing (NDT) is the examination of an object or material in any manner that will not impair its future usefulness. NDT is performed to evaluate the internal and/or external condition of materials, components and structures without any damage to them.
In the industry NDT technologies are used to help ensure the integrity and reliability of products being provided to the end users. The NDT process may range anywhere from a simple visual inspection to an intricate ultrasonic characterization of microstructures at ambient temperature or a radiography examination of parts during a manufacturing operation. Although less accurate than destructive testing, NDT is much more cost-effective and therefore widely spread in various industries including: automotive, aviation/aerospace, construction, manufacturing, nuclear engineering and petrochemicals.
Although many NDT methods are available, 6 are currently most common:                Visual Inspection—this is the most common inspection method. Work is done using fiberscopes, borescopes, magnifying glasses and mirrors. Portable video inspection unit with zoom allows inspection of large tanks and vessels, railroad tank cars, sewer lines; Robotic crawlers permit observation in hazardous or tight areas, such as air ducts, reactors and pipelines.        Liquid Penetrant Inspection—A liquid with high surface wetting characteristics is applied to the surface of the part and allowed time to seep into surface breaking defects. Then, excess liquid is removed from the surface of the part. A developer (powder) is applied to pull the trapped penetrant out the defect and spread it on the surface where it can be seen. The penetrant used is often loaded with a fluorescent dye and visual inspection is done under UV light to increase test sensitivity as the final step in the process.        Magnetic Particle Inspection—The part is first magnetized, and then finely milled iron particles coated with a dye pigment are applied to the specimen. These particles are attracted to magnetic flux leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions.        Film Radiography—The part is placed between the radiation source and a piece of photographic film. The part will stop some of the radiation. Thicker and denser area will stop more of the radiation. The film darkness (density) will vary with the amount of radiation reaching the film through the test object, revealing hidden cracks and holes.        Eddy Current Testing—uses electromagnetic induction to detect flaws in conductive materials; the Cracks cause disturbances in the magnetic field revealing any defects. This method has several limitations, among them: the surface of the material must be accessible, the finish of the material may cause bad readings, the depth of penetration into the material is limited, and flaws that lie parallel to the probe may be undetectable.        Ultrasonic Inspection (Pulse-Echo)—this method uses High frequency sound waves introduced into a material and reflected back from surfaces or flaws. The reflected sound energy is displayed versus time, and the inspector can visualize a cross section of the specimen showing the depth of features that reflect sound.        
The alternating current potential drop (ACPD) is a well-established electromagnetic technique for sizing surface-breaking defects in metals. It is particularly suited to obtaining detailed crack profiles and to monitoring crack growth or initiation. It is a contacting method which requires surface cleaning. ACPD works by inputting an alternating current into the sample so it flows across the defect. The current is normally directly injected, but can be induced. A voltage probe then measures surface potential differences.
By comparing potential differences across the crack with a reference value, the extra path length caused by the defect can be estimated, giving a value for the crack depth.
The alternating current potential drop (ACPD) technique is known in the art and is used to monitor surface cracks in electrical conductors. At high frequencies, the current flows in a superficial skin layer. Two distinct solutions are currently available for the thin and thick skin cases. However, there is no general solution that bridges these two modes in a seamless fashion. Moreover, techniques used in the art do not allow detection locating and characterization of flaws below the surface, or on the surface other than the inspected surface.
U.S. Pat. No. 5,258,708; to Sadeghi et al.; entitled “Methods and apparatus for non-destructive testing of materials with eddy currents” discloses a non-destructive method for the detection of surface cracks in metals, wherein an eddy current is induced in the surface region of a workpiece under test, at a frequency sufficiently high to generate an alternating magnetic field solely in the skin region of the workpiece. That alternating surface magnetic field is interrogated by means of a relatively small electro-magnetic induction sensor, having regard to the overall area of the induced magnetic field. The sensor provides a voltage output which is analyzed, preferably in real time, to yield an indication sensor. Also described is a probe for performing such a method.
U.S. Pat. No. 5,202,641; to Unvala Bhikhu; entitled “Method, test probe and apparatus for the measurement of alternating current potential drop by confining test current to a skin region of a test specimen”; discloses an ACPD method and system of measurement on specimen surface—using skin effect frequency and lateral current restriction by passing same current over and close to surface.
Patent GB 2161936A; to Graham at al.; entitled “alternating current potential drop crack detection”; discloses an alternating current potential drop crack detection system having differential input operational amplifier coupled to probes in test area and in reference area.
Background information related to flow of alternating current in conductors may be found in papers by Nicola Bowler: Nicola Bowler; J. Phys. D: Appl. Phys. Vol. 39 584-589 (2006); “Theory of four-point alternating current potential drop measurements on a metal half-space”; and Nicola Bowler; IEEE transactions on magnetics, vol. 41, no. 6, June 2005; “Model-Based Characterization of Homogeneous Metal Plates by Four-Point Alternating Current Potential Drop Measurements”
More background information may be found in the following references:    [1] R. Collins, W. D. Dover, and D. H. Michael, in Nondestructive Testing, edited by R. S. Sharpe (Academic, New York, 1985), Chap. 5.    [2] R. Collins, W. D. Dover, and K. B. Ranger, 13th Symposium on NDE, San Antonio, Tex., 1981.    [3] W. D. Dover, R. Collins, and D. H. Michael, Br. J. Non-Destr. Test. 33, 121 (1991).    [4] W. C. Johnson, Transmission Lines and Networks (McGraw-Hill, New York, 1950), pp. 58-80.    [5] W. D. Dover and C. C. Monahan, Fatigue Fract. Eng. Mater. Struct. 17, 1485 (1994).    [6] W. D. Dover, F. D. Charlesworth, K. A. Taylor, R. Collins, and D. H. Michael, in The Measurement of Crack Length and Shape During Fracture and Fatigue, edited by C. J. Beevers (Cradley Heath, England, 1980).    [7] O. Hirsoshi, Z. Wei, and N. A. Satya, Eng. Fract. Mech. 43,911 (1992).    [8] M. C. Lugg, NDT Int. 22, 149 (1989).    [9] D. Mirshekar-Syahkal, R. Collins, and D. H. Michael, J. Nondestruct. Eval. 3, 65 (1982).    [10] D. H. Michael and R. Collins, J. Nondestruct. Eval. 3, 19 (1982).    [11] ANSYS, Electromagnetic Toolbox [8], 2004, ANSYS Inc.